Lowe D. Networking For Dummies
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Book IV
Chapter 4
Using DNS
327
The Hosts File
Domain Description Domain Description
fi
Finland
st
Sao Tome and Principe
fm
Micronesia
su
Soviet Union
fo
Faroe Islands
to
Tonga
fr
France
tv
Tuvalu
ge
Georgia
tw
Taiwan
gov
Government
ua
Ukraine
gr
Greece
us
United States
hr
Croatia
ws
Samoa
The Hosts File
Long ago, in a network far, far away, the entire Internet was small enough
that network administrators could keep track of it all in a simple text file.
This file, called the Hosts file, simply listed the name and IP address of every
host on the network. Each computer had its own copy of the Hosts file. The
trick was keeping all those Hosts files up to date. Whenever a new host was
added to the Internet, each network administrator would manually update
his copy of the Hosts file to add the new host’s name and IP address.
As the Internet grew, so did the Hosts file. In the mid-1980s, it became obvious
that a better solution was needed. Imagine trying to track the entire Internet
today by using a single text file to record the name and IP address of the
millions of hosts on the Internet! DNS was invented to solve this problem.
Understanding the Hosts file is important for two reasons:
✦ The Hosts file is not dead. For small networks, a Hosts file may still be
the easiest way to provide name resolution for the network’s computers.
In addition, a Hosts file can coexist with DNS. The Hosts file is always
checked before DNS is used, so you can even use a Hosts file to override
DNS if you want.
✦ The Hosts file is the precursor to DNS. DNS was devised to circumvent
the limitations of the Hosts file. You’ll be in a better position to appreciate
the benefits of DNS when you understand how the Hosts file works.
The Hosts file is a simple text file that contains lines that match IP addresses
with host names. You can edit the Hosts file with any text editor, including
Notepad or by using the MS-DOS EDIT command. The exact location of the
Hosts file depends on the client operating system, as listed in Table 4-4.
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The Hosts File
Table 4-4 Location of the Hosts File
Operating System Location of Hosts File
Windows 9x/Me
c:\windows\hosts
Windows NT/2000
c:\winnt\system32\drivers\etc\hosts
Windows XP and Vista
c:\windows\system32\drivers\etc\hosts
Unix/Linux
/etc/hosts
All TCP/IP implementations are installed with a starter Hosts file. For example,
Listing 4-1 shows the sample Windows 7 TCP/IP Hosts file. As you can see,
the starter file begins with some comments that explain the purpose of the file.
The Windows 7 Hosts file ends with comments which show the host mapping
commands used to map for the host name localhost, mapped to the IP
address 127.0.0.1. The IP address 127.0.0.1 is the standard loopback
address. As a result, this entry allows a computer to refer to itself by using
the name localhost.
Note that after the 127.0.0.1 localhost entry, another localhost entry
defines the standard IPv6 loopback address (::2). This is required because
unlike previous versions of Windows, Vista provides built-in support for IPv6.
Prior to Windows 7, these lines were not commented out in the Hosts file.
But beginning with Windows 7, the name resolution for localhost is handled
by DNS itself, so its definition isn’t required in the Hosts file.
Listing 4-1: A Sample Hosts File
# Copyright (c) 1993-2009 Microsoft Corp.
#
# This is a sample HOSTS file used by Microsoft TCP/IP for Windows.
#
# This file contains the mappings of IP addresses to host names. Each
# entry should be kept on an individual line. The IP address should
# be placed in the first column followed by the corresponding host name.
# The IP address and the host name should be separated by at least one
# space.
#
# Additionally, comments (such as these) may be inserted on individual
# lines or following the machine name denoted by a ‘#’ symbol.
#
# For example:
#
# 102.54.94.97 rhino.acme.com # source server
# 38.25.63.10 x.acme.com # x client host
# localhost name resolution is handled within DNS itself.
#127.0.0.1 localhost
#::1 localhost
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Book IV
Chapter 4
Using DNS
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The Hosts File
To add an entry to the Hosts file, simply edit the file in any text editor. Then,
add a line at the bottom of the file, after the localhost entry. Each line
that you add should list the IP address and the host name that you want
to use for the address. For example, to associate the host name server1.
LoweWriter.com with the IP address 192.168.168.201, you add this line
to the Hosts file:
192.168.168.201 server1.LoweWriter.com
Then, whenever an application requests the IP address of the host name
server1, the IP address 192.168.168.201 is returned.
You can also add an alias to a host mapping. This enables users to access a
host by using the alias as an alternative name. For example, consider the
following line:
192.168.168.201 server1.LoweWriter.com s1
Here, the device at address 192.168.168.201 can be accessed as
server1.LoweWriter.com or just s1.
Listing 4-2 shows a Hosts file with several hosts defined.
Listing 4-2: A Hosts File with Several Hosts Defined
# Copyright (c) 1993-2009 Microsoft Corp.
#
# This is a sample HOSTS file used by Microsoft TCP/IP for Windows.
#
# This file contains the mappings of IP addresses to host names. Each
# entry should be kept on an individual line. The IP address should
# be placed in the first column followed by the corresponding host name.
# The IP address and the host name should be separated by at least one
# space.
#
# Additionally, comments (such as these) may be inserted on individual
# lines or following the machine name denoted by a ‘#’ symbol.
#
# For example:
#
# 102.54.94.97 rhino.acme.com # source server
# 38.25.63.10 x.acme.com # x client host
# localhost name resolution is handled within DNS itself.
# 127.0.0.1 localhost
# ::1 localhost
192.168.168.200 doug.LoweWriter.com #Doug’s computer
192.168.168.201 server1.LoweWriter.com s1 #Main server
192.168.168.202 debbie.LoweWriter.com #Debbie’s computer
192.168.168.203 printer1.LoweWriter.com p1 #HP Laser Printer
Even if your network uses DNS, every client still has a Hosts file that defines
at least localhost.
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Understanding DNS Servers and Zones
Understanding DNS Servers and Zones
A DNS server is a computer that runs DNS server software, helps to maintain
the DNS database, and responds to DNS name resolution requests from
other computers. Although many DNS server implementations are available,
the two most popular are Bind and the Windows DNS service. Bind runs
on Unix-based computers (including Linux computers), and Windows DNS
(naturally) runs on Windows computers. Both provide essentially the same
services and can interoperate.
The key to understanding how DNS servers work is to realize that the DNS
database — that is, the list of all the domains, subdomains, and host
mappings — is a massively distributed database. No single DNS server
contains the entire DNS database. Instead, authority over different parts of
the database is delegated to different servers throughout the Internet.
For example, suppose that I set up a DNS server to handle name resolutions
for my LoweWriter.com domain. Then, when someone requests the IP
address of doug.LoweWriter.com, my DNS server can provide the answer.
However, my DNS server wouldn’t be responsible for the rest of the Internet.
Instead, if someone asks my DNS server for the IP address of some other
computer, such as coyote.acme.com, my DNS server will have to pass the
request on to another DNS server that knows the answer.
Zones
To simplify the management of the DNS database, the entire DNS namespace
is divided into zones, and the responsibility for each zone is delegated to a
particular DNS server. In many cases, zones correspond directly to domains.
For example, if I set up a domain named LoweWriter.com, I can also set
up a DNS zone called LoweWriter.com that’s responsible for the entire
LoweWriter.com domain.
However, the subdomains that make up a domain can be parceled out to
separate zones, as shown in Figure 4-2. Here, a domain named LoweWriter.
com has been divided into two zones. One zone, us.LoweWriter.com, is
responsible for the entire us.LoweWriter.com subdomain. The other zone,
LoweWriter.com, is responsible for the entire LoweWriter.com domain
except for the us.LoweWriter.com subdomain.
Why would you do that? The main reason is to delegate authority for the
zone to separate servers. For example, Figure 4-2 suggests that part of the
LoweWriter.com domain is administered in the United States and that part
of it is administered in France. The two zones in the figure allow one server
to be completely responsible for the U.S. portion of the domain, and the
other server handles the rest of the domain.
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Book IV
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Using DNS
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Understanding DNS Servers and Zones
Figure 4-2:
DNS zones.
(Root)
LoweWriter.com zone
us.LoweWriter.com
zone
com
LoweWriter
us
france
The Hosts file can be the basis of a fun, prac-
tical joke. Of course, neither I nor my editors
or publishers recommend that you actually do
this. If it gets you into trouble, don’t send your
lawyers to me. This sidebar is here only to let
you know what to do if it happens to you.
The idea is to edit your poor victim’s Hosts
file so that whenever the user tries to access
his favorite Web site, a site of your choosing
comes up instead. For example, if you’re trying
to get your husband to take you on a cruise,
add a line to his Hosts file that replaces his
favorite Web site with the Web site for a cruise
line. For example, this line should do the trick:
151.124.250.181 www.espn.com
Now, whenever your husband tries to call up
the ESPN Web site, he’ll get the Carnival Cruise
Lines home page instead.
Of course, to actually pull a stunt like this would
be completely irresponsible. Especially if you
didn’t first make a backup copy of the Hosts
file, just in case it somehow gets messed up.
Be warned: If the wrong Web sites suddenly
start coming up, check your Hosts file to see
whether it’s been tampered with.
The old phony Hosts file trick
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Understanding DNS Servers and Zones
The following are the two basic types of zones:
✦ A primary zone is the master copy of a zone. The data for a primary
zone is stored in the local database of the DNS server that hosts the
primary zone. Only one DNS server can host a particular primary zone.
Any updates to the zone must be made to the primary zone.
✦ A secondary zone is a read-only copy of a zone. When a server hosts a
secondary zone, the server doesn’t store a local copy of the zone data.
Instead, it obtains its copy of the zone from the zone’s primary server by
using a process called zone transfer. Secondary servers must periodically
check primary servers to see whether their secondary zone data is still
current. If not, a zone transfer is initiated to update the secondary zone.
Primary and secondary servers
Each DNS server is responsible for one or more zones. The following are the
two different roles that a DNS server can take:
✦ Primary server for a zone, which means that the DNS server hosts a
primary zone. The data for the zone is stored in files on the DNS server.
Every zone must have one primary server.
✦ Secondary server for a zone, which means that the DNS server obtains
the data for a secondary zone from a primary server. Every zone should
have at least one secondary server. That way, if the primary server goes
down, the domain defined by the zone can be accessed via the secondary
server or servers.
A secondary server should be on a different subnet than the zone’s primary
server. If the primary and secondary servers are on the same subnet, both
servers will be unavailable if the router that controls the subnet goes down.
Note that a single DNS server can be the primary server for some zones and
a secondary server for other zones. A server is said to be authoritative for
the primary and secondary zones that it hosts because it can provide
definitive answers for queries against those zones.
Root servers
The core of DNS comprises the root servers, which are authoritative for
the entire Internet. The main function of the root servers is to provide the
address of the DNS servers that are responsible for each of the top-level
domains. These servers, in turn, can provide the DNS server address for
subdomains beneath the top-level domains.
The root servers are a major part of the glue that holds the Internet
together. As you can imagine, they’re swamped with requests day and night.
A total of 13 root servers are located throughout the world. Table 4-5 lists
the IP address and location of each of the 13 root servers.
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Book IV
Chapter 4
Using DNS
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Understanding DNS Servers and Zones
Table 4-5 The 13 Root Servers
Server IP Address Operator Location
A
198.41.0.4
VeriSign Global
Registry Services
Dulles, VA
B
192.228.79.201
Information
Sciences Institute
Marina Del Rey, CA
C
192.33.4.12
Cogent
Communications
Herndon, VA and
Los Angeles, CA
D
128.8.10.90
University of
Maryland
College Park, MD
E
192.203.230.10
NASA Ames
Research Center
Mountain View, CA
F
192.5.5.241
Internet Systems
Consortium
Palo Alto, CA; San
Jose, CA; New York
City; San Francisco;
Madrid; Hong Kong;
Los Angeles
G
192.112.36.4
U.S. DOD Network
Information
Vienna, VA Center
H
128.63.2.53
U.S. Army
Research Lab
Aberdeen, MD
I
192.36.148.17
Autonomica Stockholm
J
192.58.128.30
VeriSign Global
Registry Services
Dulles, VA;
Mountain View CA;
Sterling, VA (two
locations); Seattle,
WA; Amsterdam,
NL; Atlanta, GA; Los
Angeles
K
193.0.14.129
Reseaux IP
Europeens
Network
London
Coordination Centre
L
199.7.83.42
IANA Los Angeles
M
202.12.27.33
WIDE Project Tokyo
DNS servers learn how to reach the root servers by consulting a root hints
file that’s located on the server. In the Unix/Linux world, this file is known
as named.root and can be found at /etc/named.root. For Windows
DNS servers, the file is called cache.dns and can be found in \windows\
system32\dns\ or \winnt\system32\dns\, depending on the Windows
version. Listing 4-3 shows the file itself.
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Understanding DNS Servers and Zones
Listing 4-3: The Named.Root File
; This file holds the information on root name servers needed to
; initialize cache of Internet domain name servers
; (e.g. reference this file in the “cache . <file>”
; configuration file of BIND domain name servers).
;
; This file is made available by InterNIC
; under anonymous FTP as
; file /domain/named.root
; on server FTP.INTERNIC.NET
; -OR- RS.INTERNIC.NET
;
; last update: Dec 12, 2008
; related version of root zone: 2008121200
;
; formerly NS.INTERNIC.NET
;
. 3600000 IN NS A.ROOT-SERVERS.NET.
A.ROOT-SERVERS.NET. 3600000 A 198.41.0.4
A.ROOT-SERVERS.NET. 3600000 AAAA 2001:503:BA3E::2:30
;
; FORMERLY NS1.ISI.EDU
;
. 3600000 NS B.ROOT-SERVERS.NET.
B.ROOT-SERVERS.NET. 3600000 A 192.228.79.201
;
; FORMERLY C.PSI.NET
;
. 3600000 NS C.ROOT-SERVERS.NET.
C.ROOT-SERVERS.NET. 3600000 A 192.33.4.12
;
; FORMERLY TERP.UMD.EDU
;
. 3600000 NS D.ROOT-SERVERS.NET.
D.ROOT-SERVERS.NET. 3600000 A 128.8.10.90
;
; FORMERLY NS.NASA.GOV
;
. 3600000 NS E.ROOT-SERVERS.NET.
E.ROOT-SERVERS.NET. 3600000 A 192.203.230.10
;
; FORMERLY NS.ISC.ORG
;
. 3600000 NS F.ROOT-SERVERS.NET.
F.ROOT-SERVERS.NET. 3600000 A 192.5.5.241
F.ROOT-SERVERS.NET. 3600000 AAAA 2001:500:2F::F
;
; FORMERLY NS.NIC.DDN.MIL
;
. 3600000 NS G.ROOT-SERVERS.NET.
G.ROOT-SERVERS.NET. 3600000 A 192.112.36.4
;
; FORMERLY AOS.ARL.ARMY.MIL
;
. 3600000 NS H.ROOT-SERVERS.NET.
H.ROOT-SERVERS.NET. 3600000 A 128.63.2.53
H.ROOT-SERVERS.NET. 3600000 AAAA 2001:500:1::803F:235
;
; FORMERLY NIC.NORDU.NET
;
. 3600000 NS I.ROOT-SERVERS.NET.
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Understanding DNS Queries
I.ROOT-SERVERS.NET. 3600000 A 192.36.148.17
;
; OPERATED BY VERISIGN, INC.
;
. 3600000 NS J.ROOT-SERVERS.NET.
J.ROOT-SERVERS.NET. 3600000 A 192.58.128.30
J.ROOT-SERVERS.NET. 3600000 AAAA 2001:503:C27::2:30
;
; OPERATED BY RIPE NCC
;
. 3600000 NS K.ROOT-SERVERS.NET.
K.ROOT-SERVERS.NET. 3600000 A 193.0.14.129
K.ROOT-SERVERS.NET. 3600000 AAAA 2001:7FD::1
;
; OPERATED BY ICANN
;
. 3600000 NS L.ROOT-SERVERS.NET.
L.ROOT-SERVERS.NET. 3600000 A 199.7.83.42
L.ROOT-SERVERS.NET. 3600000 AAAA 2001:500:3::42
;
; OPERATED BY WIDE
;
. 3600000 NS M.ROOT-SERVERS.NET.
M.ROOT-SERVERS.NET. 3600000 A 202.12.27.33
M.ROOT-SERVERS.NET. 3600000 AAAA 2001:DC3::35
; End of File
Caching
DNS servers don’t really like doing all that work to resolve DNS names, but
they’re not stupid. They know that if a user visits www.wiley.com today,
he’ll probably do it again tomorrow. As a result, name servers keep a cache
of query results. The next time the user visits www.wiley.com, the name
server is able to resolve this name without having to query all those other
name servers.
The Internet is constantly changing, however, so cached data can quickly
become obsolete. For example, suppose that Wiley Publishing, Inc., switches
its Web site to a different server? It can update its name servers to reflect
the new IP address, but any name servers that have a cached copy of the
query will be out of date.
To prevent this from being a major problem, DNS data is given a relatively
short expiration time. The expiration value for DNS data is called the TTL
(Time to Live). TTL is specified in seconds. Thus, a TTL of 60 means the data
is kept for one minute.
Understanding DNS Queries
When a DNS client needs to resolve a DNS name to an IP address, it uses a
library routine — a resolver — to handle the query. The resolver takes care of
sending the query message over the network to the DNS server, receiving and
interpreting the response, and informing the client of the results of the query.
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Understanding DNS Queries
A DNS client can make two basic types of queries: recursive and iterative.
The following list describes the difference between these two query types.
(The following discussion assumes that the client is asking the server for the
IP address of a host name, which is the most common type of DNS query.
You find out about other types of queries later; they, too, can be either
recursive or iterative.)
✦ Recursive queries: When a client issues a recursive DNS query, the
server must reply with either the IP address of the requested host name
or an error message indicating that the host name doesn’t exist. If the
server doesn’t have the information, it asks another DNS server for the
IP address. When the first server finally gets the IP address, it sends it
back to the client. If the server determines that the information doesn’t
exist, it returns an error message.
✦ Iterative queries: When a server receives an iterative query, it returns
the IP address of the requested host name if it knows the address. If the
server doesn’t know the address, it returns a referral, which is simply
the address of a DNS server that should know. The client can then issue
an iterative query to the server to which it was referred.
Normally, DNS clients issue recursive queries to DNS servers. If the server
knows the answer to the query, it replies directly to the client. If not, the
server issues an iterative query to a DNS server that it thinks should know
the answer. If the original server gets an answer from the second server,
it returns the answer to the client. If the original server gets a referral to a
third server, the original server issues an iterative query to the third server.
The original server keeps issuing iterative queries until it either gets the
answer or an error occurs. It then returns the answer or the error to the
client.
A real-life DNS example
Confused? I can understand why. An example may help to clear things up.
Suppose that a user wants to view the Web page www.wiley.com. The
following sequence of steps occurs to resolve this address:
1. The browser asks the client computer’s resolver to find the IP address
of www.wiley.com.
2. The resolver issues a recursive DNS query to its name server.
In this case, I’ll call the name server ns1.LoweWriter.com.
3. The name server ns1LoweWriter.com checks whether it knows the IP
address of www.wiley.com.
It doesn’t, so the name server issues an iterative query to one of the root
name servers to see whether it knows the IP address of www.wiley.com.
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